Noise suppression in an open mr apparatus

ABSTRACT

The invention relates to an MR apparatus ( 1 ) having an examination volume ( 6 ) of the open type such that the examination volume ( 6 ) is also accessible laterally, having main coil devices ( 2 ) for producing a main magnetic field that are disposed on two opposite sides of the examination volume ( 6 ), having at least one high-frequency coil device ( 5 ) for exciting the proton precession and having at least one two-part gradient coil device ( 4 ), disposed on opposite sides of the examination volume ( 6 ), for the positional coding of the excitation of the proton precession. The dynamic Lorentz forces have hitherto resulted in a high oscillation level. It is an object of the invention to provide an MR apparatus ( 1 ) that has only a low oscillation level. The invention proposes that, on at least one side of the examination volume ( 6 ), a central recess ( 7 ) extends from the examination volume ( 6 ) and through the main coil device ( 2 ), in which recess ( 7 ) a holding element ( 9 ) is disposed to whose side facing the examination volume ( 6 ) the gradient coil device ( 4 ) is exclusively attached.

The invention relates to an MR apparatus having an examination volume ofthe open type such that the examination volume is also accessiblelaterally, having main coil devices for producing a main magnetic fieldthat are disposed on two opposite sides of the examination volume,having at least one high-frequency coil device for exciting the protonprecession and having at least one two-piece gradient coil disposed onopposite sides of the examination volume for positional coding of theexcitation of the proton precession.

A main coil device is to be understood as meaning both a conventionalcoil device and a superconducting coil device which is cooled to a lowtemperature, and whose thermally insulating casing is also referred tobelow as a cryostat.

Within the scope of a typical magnetic resonance imaging method, themagnetic moment of the protons is aligned in one spatial direction bymeans of a strong stationary magnetic field of, for example, 1.5 Tesla.The individual protons are excited to precess by means of shortelectromagnetic, high-frequency pulses and then align themselves againaccording to the external, strong magnetic field. In this connection, inparticular, the excitation and relaxation times and also the frequenciesof the precessions are tissue-dependent and, within the scope of themeasurement, provide, together with a positional coding of theexcitation, information about the spatial arrangement of varioustissues. The positional coding makes use of position-dependentfrequencies and phases of the precession excitation and makes itpossible to draw a conclusion about the position of the respectiveemission by means of a Fourier transformation of the measured MR signal.

The coils used for the precession excitation of the protons are, as arule, situated in the examination space surrounded by the remainingcoils.

The examination volume of known MR apparatus is generally designed as acylindrical tube into which the patient is introduced from an axial end.The closed and constricted conditions during the examination in aconventional MR apparatus bring about an unpleasant and uncomfortablefeeling in many patients and claustrophobic-type reactions in some. Forthis reason, the latest developments are aimed at providing an open MRapparatus. The patient lies on an examination table that contains coilsor magnetic devices that interact with coil devices or magnetic devicesin a roofing upper section about one half meter from the table top. Thesupporting area and the roofing upper section have, as integralcomponents, the main magnets provided for aligning the protons, thegradient coils provided for the positional coding and the high-frequencycoils used for the excitation.

The strong magnetic fields produce, beneath the individual coils andmagnets, high forces that have to be supported. In addition to thestatic forces, dynamically varying forces also occur and these may causesevere oscillations and a high noise level. Since the composite tube ofa closed MR apparatus is unnecessary in the open type, supporting thehigh static and dynamic forces is particularly problematical.

In the prior art, the gradient coils are generally mounted in a planarconfiguration by means of suitable attachment elements on that surfaceof the main coil device that points in the direction of the examinationvolume. The Lorentz forces that occur dynamically during operationresult in a high oscillation level in this type of attachment, since thegradient coils cause the entire MR apparatus to vibrate due to theirattachment to the main coil or its housing. Particularly high noiselevels are produced in this connection by oscillation excitations of thecryostats that are generally used and that are surrounded by a housingforming a vacuum chamber. The main emphasis in the housing design isgenerally to avoid heat bridges, for which reason the housing isextremely susceptible to oscillation. The oscillations not only affectthe well-being of the patient adversely, but they also impair thequality of the magnetic fields, which is accompanied by a serious lossin the image quality.

Proceeding from the disadvantages and problems of the prior art, it isan object of the invention to provide an MR apparatus of the open typewith lateral accessibility and features an very low oscillation levelonly and low noise emissions even at high magnetic field strengths andfast switching sequences.

According to the invention, the object is achieved by an MR apparatus ofthe type mentioned at the outset in which a central recess extends fromthe examination volume through the main coil device on at least one sideof the examination volume, and in the recess a holding element isdisposed to whose side facing the examination volume the gradient coildevice is exclusively attached.

A decisive advantage of the attachment according to the invention of thegradient coil devices resides in the extremely far-reaching mechanicaldecoupling of the main coil device. The main coil device can, therefore,no longer be excited to oscillate by oscillations and vibrations of thegradient coil device. The operation of the MR apparatus is,consequently, significantly quieter, which is conducive to thewell-being of the patient to be examined and reduces the risk ofclaustrophobic states in the patient.

The low oscillation level of the MR apparatus according to the inventionhas, in addition, the advantage that the magnetic fields needed forimaging can be produced without interference and with a high quality,which improves the image quality of open-type MR apparatus.

An advantageous further development of the invention makes provisionthat the holding element is designed as a column. In this case, it isexpedient to attach the gradient coil device to the axial end of theholding element that faces the examination volume. A column-type designof the holding element fits in with the internal structure of the maincoil device and the arrangement of a central recess in the main coildevice, since the formation of the main magnetic field in the region ofthe central recess does not necessarily require components of the maincoil device.

Advantageously, the holding element extends in this case through out theoverall depth of the main coil device and is either attached to a rigidsupporting frame or to the main coil device in the central recess. Ifthe holding element is attached in the central recess of the main coildevice, it is expedient if the main coil device or the housing of themain coil device is appropriately stiffened and reinforced or thehousing of the main coil device is stiffened by the joint to the holdingelement. If the main coil device is surrounded by a housing made, forexample, of steel, the rigidity of the housing is increased by theformation of a central recess and the continuation of the housing intothe central recess. The increasing rigidity brings about a lowersusceptibility to oscillation excitations.

The attachment of the gradient coil device becomes particularly robustand immune to oscillations if the holding element is designed as aconical column and the wider end of the holding element is attached toan external supporting frame. The moment of resistance of the column isthus optimally adapted to the bending-moment loading that occurs.

To reduce the oscillation level and the noise development further, it isadvantageous if piezoelectric or hydraulic actuators, by means of whichthe oscillation level can be actively controlled, are situated betweenthe holding element and the gradient coil device.

An activation of the actuators that is dependent on the respectiveoperating state makes it possible to compensate for a large part of theoscillations that occur. In this connection, various operating statescan be matched to an optimum oscillation behavior beforehand byoptimizing the frequency of the activation of the actuators beforehandby simulation or experimentally. In this case, the actuators may also beprovided at the end of the holding element that points away from thegradient coil device, for example, between the holding element and arigid supporting frame or the main coil device.

An advantageous further development of the invention makes provisionthat the actuators are activated as a function of the coil current inthe gradient coil devices. In this connection, it is expedient toconfigure the respective position of the individual actuators as afunction of an appropriately weighted sum of the current in the gradientcoil devices for the individual spatial directions.

In particular, in the case of a housing of the main coil device made ofmetal or steel, it is expedient if at least a part of the stray field,but preferably the entire stray field of the gradient coil device, isactively shielded by shielding coils. In this case, the shielding coilsare preferably constructed as integral components of the gradient coildevices. In this way, an excitation of the housing of the main coildevice because of the Lorentz forces produced by the magnetic field canbe avoided.

An advantageous further development of the invention makes provisionthat the gradient coils of at least one spatial direction are jointlyattached to a supporting frame. In this case, it is expedient if thesupporting frame has no direct mechanical coupling to the main coildevice. In this way, the main coil device remains unaffected by amechanical oscillation excitation from the gradient coil devices and theimage quality and also the noise level are decisively improved.

An equally inexpensive as well as long-lived and robust design of the MRapparatus according to the invention makes provision that dampingelements are disposed in the recess between the holding element and themain coil device. Cylindrical or annular three-dimensional moldings ofelastomeric materials may, for example, be provided as damping elements.

In particular, if the holding element is attached in the central recessof the main coil device, it is expedient if the main coil device is ofreinforced design in the region of the central recess. For example,ribbing of the housing of the main coil device or a sleeve thatreinforces the recess may serve as reinforcement.

These and other aspects of the invention are apparent from and will beelucidated with reference to the embodiments described hereinafter.

IN THE DRAWINGS

FIGS. 1 to 3 show cross sections through MR apparatus having anattachment according to the invention of the gradient coil devices bymeans of holding elements on the housing of the main coil device,

FIG. 4 shows a cross section through the lower part of an MR apparatushaving a gradient coil device that is attached to a holding elementaccording to the invention, which holding element is attached to thehousing of a main coil device,

FIGS. 5 to 7 show an MR apparatus having a holding element according tothe invention on which a gradient coil device is attached and which ismounted on an external supporting plane.

In each of FIGS. 1 to 7, the MR apparatus 1 is denoted in its entiretyby the reference symbol 1. Essential components of the MR apparatus 1are two coupled cryostats 3, each of which accommodates a main coildevice 2 that has various coils 2 a that produce the magnetic fieldneeded. The individual coils 2 a are provided for producing a powerfulmagnetic field, constant to the greatest possible extent, for thepurpose of aligning the protons and are cooled, by means of thecryostats 3, to a temperature suitable for superconduction. Thecryostats 3 are each surrounded by a housing 8. Situated in the interiorof the cryostats 3, adjacent in each case to the housing wall, is avacuum chamber 3 a that serves to thermally insulate the main coildevices 2 that operate at low temperature.

-   -   The gradient coil devices 4 adjoining the examination volumes 6        produce a locally variable magnetic field for the position        coding of the proton excitation, and high-frequency coil devices        5 provide for the excitation of the proton precession. The two        main coil devices 2 are situated underneath a laterally        accessible examination volume 6. The two main coil devices 2        each have a central recess 7 that generally extends vertically        through the entire main coil device 2. The main coil devices 2        are each situated in the housing 8 of the cryostat 3 that        annularly surrounds the central recess 7 in each case. Situated        in each of the central recesses 7 situated above and below is a        respective holding element 9.

In the immediate vicinity of the examination volume 6 the high-frequencycoil device 5 of flat design is situated between each time theexamination volume 6 and the central recesses 7. Disposed immediatelyadjacent to each high-frequency coil device 5 is the gradient coildevice 4, which is of flat design for the surface pointing in thedirection of the examination volume 6 and conically designed for thesurface pointing away from the examination volume 6. To match theconical shape of the gradient coil device 4, the housing 8 of thecryostat 3 is also of conical design at the opening of the centralrecess 7 into the examination volume 6.

The main coil device 2 or the housing 8 surrounding the cryostat 3 is ineach case manufactured from an approximately 8 mm thick steel sheet,which ensures adequate ruggedness for absorbing the vacuum load.

The gradient coil devices 4 are each attached to a holding element 9situated in the central recess 7.

In the exemplary embodiment shown in FIG. 1, the holding elements 9 aredesigned as columns and mounted on the housings 8 of the cryostats 3 bymeans of damping elements 10. The mounting takes place exclusively inregions of increased rigidity of the housing 8 of the cryostats 3. Thedamping elements 10 are constructed here as rings that are composed ofan elastic material suitable for damping. The column-shaped holdingelements 9 are each provided with a central bore 11 that has alongitudinal extension parallel to the central recess 7. The holdingelements 9 are of a modular construction such that a screw joint 12makes splitting into two parts 9 a, 9 b possible in each case. That part9 a of the holding element 9 that is situated at the side of thegradient coil device 4 is in each case firmly joined to the gradientcoil device 4. A bolt 13 of the screw joint 12 extends in the directionof the examination volume 6, through the gradient coil device 4, and isin each case joined to the cryostat 3.

Provided between the gradient coil devices 4 and the housings 8 of thecryostat 3 in each case is a gap 14 of at least 2 cm in order toreliably avoid mechanical coupling or transmission of oscillations. Inorder to ensure that no oscillations can be transmitted in each casebetween the high-frequency coil device 5 and the gradient coil device 4,a gap 15 is also provided between these two components.

The holding element 9 is in each case joined to the housing 8 of thecryostat 3 only by means of two annular damping elements 10 andotherwise has sufficient clearance from the boundary walls of thecentral recess 7 so that no mechanical coupling of any kind can occurhere either.

In the exemplary embodiment shown in FIG. 2, the holding elements 9 areeach of modular structure and, when disassembled, split up into a part 9a facing the examination volume 6 and a part 9 b situated further fromthe examination volume 9.

The exemplary embodiment in FIG. 3 proposes a similar design of theholding element 9. Here, the holding elements 9 are each likewise splitinto two parts 9 a, 9 b, the part 9 b of the holding element 9, which isfurther away from the examination volume in each case, being designedeach time as an integral component of the housing 8 of the cryostat 3.Here again, each gradient coil device 4 is attached, together with thecryostat 5, by means of a screw joint 12 to that part 9 a of the holdingelement 9 that is nearer the examination volume 6. The housings 8 of thecryostats 3 are each reinforced by means of the parts 9 b of the holdingelements 9 in that region of the central recess 7 that is further awayfrom the examination volume 6.

In the case of the exemplary embodiment shown in FIG. 4, the holdingelement 9 is attached to the lower housing 8 of the main coil device 2by means of annular receptacles 20 that each have annular elasticinserts 21 for damping. The holding element 9 is each time attached tothe cryostat 3 by radial bolts which are not shown.

For the purpose of energy supply and cooling of the gradient coil device4, the holding element 9 is passed through by various leads 22 that arerouted through bores 23 that extend in the longitudinal direction of theholding element 9. On that side of the holding element 9 that points inthe direction of the examination volume 6, the leads 22 open intoelectrical contacts 24 that correspond to electrical contacts 25 on thegradient coil device 4. The gradient coil device 4 is attached by meansof a screw joint 12 and a radial guide 26 to the holding element 9.

In the case of the exemplary embodiments of MR apparatus 1 shown in theFIGS. 5, 6 and 7, holding elements 9 according to the invention for thetwo gradient coil devices 4 are attached to an external supporting frame27. The supporting frame 27 is provided with its own mounting elements28 so that no direct mechanical coupling of any kind occurs between thecryostat 3 and the gradient coil device 4.

In FIG. 5, the truncated cone that is remote from the outer supportingframe 27 and has a steeper cone angle is partly surrounded by thegradient coil device 4. Situated in the region between the gradient coildevice 4 and the cryostat are equalizing elements 35 for supporting thegradient coil 4.

In the diagram of FIG. 6 it can be clearly seen that the outersupporting frame 27 is structurally separated from the supportingstructure of the cryostat 3.

The holding element 9, which is of column-shaped design in each case, isof conical design in one region and is attached by one end 29 to theexternal supporting frame 27. The holding element 9 has, in this case,the shape of two truncated cones that stand on one another by means oftheir narrow end, the truncated cone that is joined to the outersupporting frame 27 being less sharply conically shaped.

In each of the MR apparatus 1 of the FIGS. 5 and 7, there are disposedbetween the gradient coil device 4 and the holding element 9piezoelectric actuators 30 that serve to actively control theoscillation level. The actuators 30 are activated as a function of theoperating state of the gradient coil device 4 and are designed for aminimum oscillation level.

In the embodiment shown in FIG. 6, the gradient coil device is attachedto the holding element 9 by means of a plurality of screw bolts 31. Theholding element 9 is of conical design almost over its entirelongitudinal extension.

The holding element shown in FIG. 7 extends into the gradient coildevice with a head of hammer-shaped cross section.

1. An MR apparatus (1) having an examination volume (6) of the open typesuch that the examination volume (6) is also accessible laterally,having main coil devices (2) for producing a main magnetic field thatare disposed on two opposite sides of the examination volume (6), havingat least one high-frequency coil device (5) for exciting the protonprecession and having at least one two-piece gradient coil device (4)disposed on opposite sides of the examination volume (6) for positionalcoding of the excitation of the proton precession, characterized in thata central recess (7) extends from the examination volume (6) through themain coil device (2) on at least one side of the examination volume (6),and that in the recess (7) a holding element (9) is disposed to whoseside facing the examination volume (6) the gradient coil device (4) isexclusively attached.
 2. An MR apparatus (1) as claimed in claim 1,characterized in that the holding element (9) is designed as a column.3. An MR apparatus (1) as claimed in claim 1, characterized in that theholding element (9) is designed as a conical column and the wider end ofthe holding element (9) is attached to an external supporting frame(27).
 4. An MR apparatus (1) as claimed in claim 1, characterized inthat piezoelectric or hydraulic actuators (30), by means of which theoscillation level can be actively controlled, are disposed between theholding element (9) and the gradient coil device (4).
 5. An MR apparatus(1) as claimed in claim 1, characterized in that at least a part of thestray field of the gradient coil device (4) is actively shielded bymeans of shielding coils.
 6. An MR apparatus (1) as claimed in claim 1,characterized in that the gradient coils of at least one spatialdirection are jointly attached to a supporting frame (27) that has nodirect mechanical coupling to the main coil device (2).
 7. An MRapparatus (1) as claimed in claim 1, characterized in that dampingelements are disposed in the recess (7) between the holding element (9)and the main coil device (2).
 8. An MR apparatus (1) as claimed in claim1, characterized in that the holding element (9) is attached to the maincoil device (2) in the central recess (7).
 9. An MR apparatus (1) asclaimed in claim 8, characterized in that the main coil device (2) is ofreinforced design in the region of the central recess (7).